Laser modulation control method and apparatus

ABSTRACT

Laser modulation control apparatus and methods which provide direct control of the transmitted optical extinction ratio of a semiconductor laser. A relatively low frequency and low amplitude pilot tone is superimposed on the signal used to drive the laser. Setting the amplitude of the pilot tone to a fixed fraction of the laser modulation current causes the transmitted optical power to vary a fixed fraction of the optical data amplitude at the pilot tone frequency. By using feedback to control the laser modulation current, the amplitude of the variation can be maintained at a desired value, which in turn maintains the transmitted optical data amplitude at a constant value, regardless of variations due to operating temperature or laser aging. A separate control loop is employed to maintain the average optical power at a fixed value. Since the optical data amplitude and the average optical power remain constant, the optical extinction ratio is also constant. Alternate embodiments are disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of semiconductor laser modulationcontrol.

2. Prior Art

Semiconductor laser modulation control is desired because the opticalcharacteristics of semiconductor lasers typically used for optical datacommunications exhibit strong temperature dependence and long termdegradation due to aging. Control can be achieved by maintaining thelaser at a fixed temperature using a thermoelectric cooler. However thisis an expensive solution, and generally will exhibit a substantial timeto settle on first turn-on. It also does not compensate for variationsdue to aging. High frequency methods have also been used to providemodulation control ("Simultaneous Feedback Control of Bias andModulation Currents for Injection Lasers," Chen, F. S., ElectronicsLetters, Vol. 16, Pgs. 7-8, 1980; U.S. Pat. No. 5,402,433, Stiscia, J.J., 1995), but these techniques require expensive high speedphotodiodes, careful control of circuit parasitics, and complex, highfrequency circuits with high power dissipation. Therefore, techniqueshave been implemented ("Laser Level-Control Circuit for High-Bit-RateSystems Using a Slope Detector," Smith, D. W., Electronics Letters, Vol.14, Pgs. 775-776, 1978; U.S. Pat. No. 4,385,387, Trimmel, H., 1981;"Laser Diode Modulation and Noise," Petermann, K., Kluwer AcademicPublishers, Pgs. 300-302, 1991; U.S. Pat. No. 5,557,445, Misaizu) bysuperimposing a low frequency pilot tone onto the laser current anddetecting the nonlinearities associated with the lasing threshold. Thelaser is then operated with the optical zero level at or near thethreshold. Operating near the threshold is not desirable due toincreased laser noise, turn-on delay, and chirp. Improvements of thistechnique have been made by Burley (U.S. Pat. No. 4,995,045), but themethod directly controls the optical data amplitude. Burley's circuitincludes additional components to avoid multiple operating points orlatch-up, but the transfer function of modulation current to rippleamplitude remains non-monotonic. Recent methods patented by Ries (U.S.Pat. Nos. 5,394,416 and 4,924,470) provide direct control of extinctionratio by measuring the intermodulation products of two low level pilottones superimposed on the laser current. Ries discusses the merits ofseveral automatic modulation control techniques, including pilot tonemethods (see U.S. Pat. No. 5,394,416 for a more detailed discussion).

In the present invention, due to the fact that the pilot tone isinserted as part of the modulation current, and is a fixed fraction ofthe modulation current, Ries' objections about additional high speedcircuitry and difficult phase matching are overcome. The method used byRies does provide direct control of extinction ratio, but the circuit isfar more complex than the present invention.

BRIEF SUMMARY OF THE INVENTION

The present invention is laser modulation control apparatus and methodswhich provide direct control of the transmitted optical extinction ratioof a semiconductor laser. In accordance with the invention, a relativelylow frequency and low amplitude pilot tone is superimposed on the signalused to drive the laser. By setting the amplitude of the pilot tone tobe a fixed fraction of the laser modulation current, the transmittedoptical power varies a fixed fraction of the optical data amplitude atthe pilot tone frequency. By using feedback to control the lasermodulation current, the amplitude of the variation can be maintained ata desired value, which in turn maintains the transmitted optical dataamplitude at a constant value, regardless of variations due to operatingtemperature or laser aging.

A separate control loop is employed to maintain the average opticalpower at a fixed value. Since the optical data amplitude and the averageoptical power remain constant, the optical extinction ratio is alsoconstant. Because low frequency signal processing is used to determinethe high frequency data amplitude, the transmitter bit rate is notlimited by the automatic modulation control circuitry. An advantage ofthis method is that the absolute amplitude of the pilot tone is notimportant since the technique relies on the relative amplitude betweenthe pilot tone and the modulation current. Therefore, there is no needfor precision amplitude control of the pilot tone oscillator.

The present invention is further refined by applying the pilot tone insuch a way that the amplitude of the optical power variation remainsmonotonic as the modulation current is increased beyond the lasingthreshold. By keeping this transfer function monotonic, multipleoperating points or latch-up are not possible.

Therefore, objectives of the present invention include providingcircuits and methods which provide laser modulation control at high bitrates and:

1. Do not require high speed photodiodes and detection circuitry

2. Avoid the laser threshold region because of degraded laserperformance in this region

3. Provide direct control of extinction ratio by maintaining a constantoptical data amplitude

4. Do not require additional high speed circuitry for the injection ofthe pilot tone

5. Minimize circuit complexity

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of one embodiment of the presentinvention.

FIG. 2 graphically illustrates the relationship between laser currentsand monitor diode currents.

FIG. 3 is a schematic diagram of an alternate embodiment of the presentinvention for a laser having an AC coupled laser modulation current.

FIG. 4 graphically illustrates the relationship between laser currentsand monitor diode currents for the AC coupled circuit of FIG. 3.

FIG. 5 shows modifications of FIG. 1 including synchronous detection andinjection of the AC reference at the monitor diode input (an alternateembodiment).

FIG. 6 is another alternate embodiment which incorporates improvementsof both FIGS. 3 and 5.

DETAILED DESCRIPTION OF THE INVENTION

First referring to FIG. 1, a schematic diagram of one embodiment of thepresent invention may be seen. As shown therein, with the switch open asshown, a semiconductor LASER is driven with a bias current IBIAS, plus amodulation current Imod dependent on the state of a double ended inputdata signal D+, D-. If the data is a logic one (a higher voltage oninput D+ holding transistor Q1 on, pulling the common emitter connectionof transistors Q1 and Q2 up so that the lower voltage on input D- holdstransistor Q2 off), the modulation current Imod will also pass throughthe laser as Q1 conducts. If the data is a logic zero, transistor Q1will be off and transistor Q2 will be on. This directs Imod from thepower supply VCC through transistor Q2, rather than through the laserand transistor Q1.

As may be seen in FIG. 1, an additional current Imod/X will be switchedinto the emitters of transistors Q1 and Q2, depending on the state ofthe relatively low frequency oscillator. The laser current is composedof IBIAS during zero data bits. During one data bits, the laser currentis an envelope having a lower value of Imod+ IBIAS and a higher value ofImod+ IBIAS+ Imod/X, giving an average value of Imod+ IBIAS+ Imod/2X andan amplitude of the envelope variation of Imod/X. The minimum differencein the laser current between a logic zero and a logic one is Imod, andthe average difference in the laser current between a logic zero and alogic one is Imod+ Imod/2X=Imod(1+1/2X)

If the laser operates exclusively in the linear region above thethreshold, the optical signal emitted by the laser will have a zerolevel dependent on IBIAS, unaffected by the low frequency switching ofImod/X through transistor Q1. The difference in the optical outputbetween a logic zero and a logic one input, whether considering theminimum difference or the average difference, will be proportional toImod, and the variation in the optical output for a logic one input dueto the low frequency switching of Imod/X through transistor Q1 will beproportional to Imod/X, which of course is proportional to Imod itself.Thus control of Imod to obtain a predetermined low frequency variationin the optical output of the laser simultaneously provides apredetermined difference in the optical output between a logic zero anda logic one input, without any high frequency control circuits andtechniques being used. Also measuring the average optical output of thelaser and controlling IBIAS in response to drive the average measuredoutput to the desired output, the average optical output and theextinction ratio are directly controlled by simple, low frequencycircuits.

In the foregoing overview of the function of the circuit of FIG. 1, itis apparent that it is important to be able to maintain the currentsource Imod/X at a fixed fraction 1/X of the current source Imod. Thisis easily done however, by use of a current mirror using transistors ofdifferent emitter area so that the current mirrored is a fixed fractionof the current in the mirroring transistor.

Having now given an overview of the purpose of the circuit of FIG. 1,the details thereof will now be described. A monitor photodiode MDdetects a portion of the emitted optical signal from the laser and emitsa photocurrent. The high frequency (data) information in thephotocurrent is filtered by capacitor Cmd such that the remainingcurrent is a DC current with the low frequency ripple. The desired DCmonitor diode current is subtracted from the monitor diode current by areference current source Imd. Transimpedance amplifier A1 with feedbackresistor Rf converts the residual current (monitor photodiode currentminus the current of reference current source Imd) into a voltage whichis coupled to an automatic power control (APC) error amplifier A2 and alow frequency amplitude detector.

The error amplifier A2 has a positive gain, a direct coupled low passoutput, and a cut off frequency set by compensation capacitor Capc at afrequency lower than that of the low frequency oscillator. With thenegative feedback of amplifier A1, the output of amplifier A2 controlsthe value of Ibias such that the DC monitor photodiode current is equalto the desired current Imd. Thus the average optical signal emitted bythe laser is held to a level providing an average illumination to themonitor photodiode to provide a current Imd there through. Note that therelationship between the laser current and the laser emission ismonotonic, as is the relationship between the laser emission and themonitor current. Thus the average power control (APC) control loop has asingle, easily stabilized operating point.

If the automatic power control (APC) loop described above is closed, theoutput of amplifier A1 will be a square wave at the low frequencyoscillator frequency with amplitude ΔImd. The low frequency amplitudedetector senses the amplitude of the low frequency ripple on the outputof amplifier A1 and in response, couples a DC signal to the automaticmodulation control (AMC) error amplifier A3 proportional to the ripple.For this purpose, the low frequency amplitude detector may be, by way ofexample, a rectifying circuit to provide a direct coupled signalproportional to the ripple in the output of amplifier A1.

Amplifier A3 compares the detected ripple amplitude ΔImd with thedesired value as set by AMCREF (a DC reference voltage). Again usingnegative feedback, the output of A3 adjusts the modulation current Imodsuch that the ripple amplitude ΔImd is equal to AMCREF. The negativefeedback in this case is not provided by amplifier A1, as the amplitudeof the ripple as detected by the low frequency amplitude detector is notphase sensitive. Instead, the negative feedback is provided by amplifierA3, the input connections as shown in FIG. 1 decreasing Imod forincreasing detected ripple. Capacitor Camc sets the frequency responseof the AMC loop lower than the frequency response of the APC loop. Inthis manner, the APC loop always sets the DC optical power before theAMC loop sets the amplitude of the optical data signal, though of courseboth responding much faster than laser drift from temperature and agingvariations of the laser.

Since the amplitude of the pilot tone current ripple during logic onebits is a known fixed fraction of the modulation current Imod, theamplitude of the optical power detected by the monitor photodiode is aknown fixed multiple of the low frequency optical ripple amplitude ΔImd.The low frequency photodiode current is given as follows: ##EQU1## whereAImd is the amplitude of the low frequency ripple detected at themonitor diode, which is proportional to the transmitted amplitude of theoptical signal, ηtotal is the total slope efficiency from laser currentto monitor diode current above threshold, ImdAC is the amplitude of thedetected photodiode current, and X is the fixed fraction of modulationcurrent to pilot tone current. The factor of two must be includedbecause the photodiode detects the average optical signal. Since thelogic zero bits are not modified by the pilot tone, the average powervaries at half of the envelope amplitude. FIG. 2 graphically illustratesthe relationships described above. In that Figure, the laser currentenvelope and the monitor diode current envelope epresent the envelopes,or upper ("1") and lower ("0") values of the high frequency data signal,which is too high in frequency to itself be illustrated in the Figure.

As mentioned above, this arrangement results in a transfer function ofmodulation current to ΔImd which remains monotonic, even if Imod isincreased beyond the lasing threshold. This technique could be appliedat IBIAS, resulting in a data envelope with ripple on both the logic oneand logic zero. However, if the modulation current is increased beyondthe lasing threshold, the low frequency ripple amplitude will decreaseuntil the ripple on the logic zero is fully clipped. The may give thetransfer function multiple possible operating points.

If the present invention is applied to a laser which has an AC coupledmodulation current, such as capacitor Cac of FIG. 3, it is desirable toadjust the pilot tone frequency such that it remains above the cutoff ofthe AC coupling network. However, with this arrangement, a small amountof ripple will be apparent on the optical zero level due to droop of theAC coupling network. If this happens, multiple operating points orlatch-up may still be a problem. If a small pilot tone Imod/Y with anamplitude equal to a fixed fraction Y of the modulation current is addedwith a 180 phase shift to IBIAS, this can be avoided by making thetransfer function monotonic once again. Y can be set to 10% of X orsmaller, as this is a second order effect. This modification is shown inthe schematic of FIG. 3. As shown therein, a component of current Imod/Yis generated in addition to Imod/X. As these current components aregenerated by the circuit shown, they are in phase with each other.However, the current mirror shown reverses the phase of Imod/Y,converting the same to a true current source when Imod, as shown,actually acts as a current sink ("current source" is the phrase normallyused in a general sense for both current sources and current sinks,current sources providing a current to another circuit and current sinksreceiving or drawing a current from another circuit).

Further modification can be made without changing the spirit of thisinvention. For example, the low frequency amplitude detector can be apeak detector or a synchronous amplitude detection circuit. Also, asignal representative of the desired monitor diode ripple signal ΔImdcan be directly subtracted from the monitor diode current at the inputof amplifier A1. This is illustrated in FIG. 5, which is a Figuresimilar to FIG. 1, but with the modification described, and FIG. 6,which is a Figure similar to FIG. 3, but with the modificationdescribed. In these cases, synchronous detection is required, and theinverting input of amplifier A3 should be connected to ground instead ofa reference as described above. Thus while the present invention hasbeen disclosed and described with respect to certain preferredembodiments, it will be understood to those skilled in the art that thepresent invention may be varied without departing from the spirit andscope of the invention.

What is claimed is:
 1. A method of controlling a semiconductor laserhaving an average laser drive modulated by a laser drive signal toprovide an optical signal comprising:superimposing a pilot tone on thelaser drive signal, the pilot tone being proportional to the laser drivesignal; sensing part of the optical signal from the laser to provide ameasure of the average optical output of the laser and a measure of thevariation in the optical output of the laser caused by the pilot tone;controlling the average laser drive responsive to the average opticaloutput of the laser to provide an average laser output; and, controllingthe laser drive signal responsive to the variation in the optical outputof the laser caused by the pilot tone to provide a predeterminedvariation in the optical output of the laser caused by the pilot tone,the controlling the laser drive signal being separate from thecontrolling the average laser drive.
 2. The method of claim 1 whereinthe control of the average laser drive to provide a predeterminedaverage laser output operates with a time constant that is shorter thanthe control of the laser drive signal to provide a predeterminedvariation in the optical output of the laser caused by the pilot tone.3. The method of claim 1 wherein the step of controlling the laser drivesignal to provide a predetermined variation caused by the pilot tone isperformed by sensing the amplitude of the optical signal from the lasercaused by the pilot tone and controlling the laser drive signal based ona comparison of the last named amplitude to a reference amplitude. 4.The method of claim 1 wherein the step of controlling the laser toprovide a predetermined average laser output comprises controlling theminimum laser drive to provide a predetermined average laser output. 5.The method of claim 1 wherein the pilot tone has a frequency which issubstantially less than the frequency of the laser drive signal.
 6. Themethod of claim 5 wherein the pilot tone has an amplitude that is notmore than approximately 10% of the amplitude of the laser drive signal.7. A method of controlling a semiconductor laser having an average laserdrive modulated by a laser drive signal to provide an optical signalcomprising:providing a first controllable current through the laser;providing a second controllable current through the laser, the secondcurrent being modulated between on and off conditions by an inputsignal; providing a third current proportional to the second currentthrough the laser, the third current also being modulated between on andoff conditions by the input signal, the third current also beingmodulated between on and off conditions by a pilot tone, the pilot tonehaving a frequency which is less than the frequency of the input signal;sensing part of the optical signal from the laser to provide a measureof the average optical output of the laser and to provide a measure ofthe variations in the optical output of the laser caused by the pilottone; controlling the first current to provide a predetermined averageoptical signal from the laser; and, controlling the second current toprovide a predetermined variation in the optical signal from the lasercaused by the pilot tone.
 8. The method of claim 7 wherein the controlof the first current to provide a predetermined average optical signaloperates with a time constant that is shorter than the control of thesecond current to provide a predetermined variation in the opticalsignal from the laser caused by the pilot tone.
 9. The method of claim 7wherein the step of controlling the second current to provide apredetermined variation in the optical signal from the laser caused bythe pilot tone is performed by sensing the amplitude of the opticalsignal from the laser caused by the pilot tone and controlling thesecond current based on a comparison of the last named amplitude to areference amplitude.
 10. The method of claim 7 wherein the step ofcontrolling the first current to provide a predetermined average opticalsignal from the laser comprises controlling the minimum current throughthe laser to provide a predetermined average optical signal from thelaser.
 11. The method of claim 7 wherein the third current has anamplitude that is not more than approximately 10% of the amplitude ofthe second current.
 12. The method of claim 7 wherein the second andthird currents are capacitively coupled to the semiconductor laser. 13.The method of claim 12 further comprising a fourth current proportionalto the third current, the fourth current being coupled across thesemiconductor laser responsive to the pilot tone with a 180 phase shiftwith respect to the third current across the semiconductor laser asmodulated responsive to the pilot tone.
 14. A semiconductor laser andlaser control system comprising:a semiconductor laser; a firstcontrollable current source biasing the semiconductor laser; a secondcontrollable current source selectively switchable across thesemiconductor laser responsive to an input signal; a third currentsource proportional to the second current source and selectivelyswitchable across the semiconductor laser responsive to a pilot tone ofa frequency substantially lower than the frequency of the input signaland responsive to an input signal; a sensor sensing part of the laseremission and providing a sensor output responsive thereto; a firstcircuit responsive to the average sensor output controlling the firstcurrent source to maintain a predetermined average laser emission; and,a second circuit responsive to the variations in the sensor output atthe pilot tone frequency controlling the second current source tomaintain a predetermined laser emission at the pilot tone frequency. 15.The semiconductor laser and laser control system of claim 14 wherein thethird current source is in parallel with the second current source andis also selectively switchable across the semiconductor laser responsiveto an input signal.
 16. The semiconductor laser and laser control systemof claim 14 wherein the first circuit operates with a time constant thatis shorter than the time constant of the second circuit.
 17. Thesemiconductor laser and laser control system of claim 14 wherein thethird current source is not more than approximately 10% as large as thesecond current source.
 18. The semiconductor laser and laser controlsystem of claim 14 wherein the second and third current sources arecapacitively coupled to the semiconductor laser.
 19. The semiconductorlaser and laser control system of claim 18 further comprising a fourthcurrent source proportional to the third current source, the fourthcurrent source being switchable across the semiconductor laserresponsive to the pilot tone with a 180 phase shift with respect to thethird current source as switchable across the semiconductor laserresponsive to the pilot tone.
 20. A semiconductor laser control systemcomprising:a first controllable current source for biasing asemiconductor laser; a second controllable current source selectivelyswitchable across a semiconductor laser responsive to an input signal; athird current source proportional to the second current source andselectively switchable across a semiconductor laser responsive to apilot tone of a frequency substantially lower than the frequency of theinput signal and responsive to an input signal; a sensor for sensingpart of the emission of a semiconductor laser and providing a sensoroutput responsive thereto; a first circuit responsive to the averagesensor output controlling the first current source to maintain anaverage sensor output; and, a second circuit responsive to thevariations in the sensor output at the pilot tone frequency controllingthe second current source to maintain a predetermined sensor output atthe pilot tone frequency.
 21. The semiconductor laser control system ofclaim 20 wherein the third current source is in parallel with the secondcurrent source and is also selectively switchable across a semiconductorlaser responsive to an input signal.
 22. The semiconductor laser controlsystem of claim 20 wherein the first circuit operates with a timeconstant that is shorter than the time constant of the second circuit.23. The semiconductor laser control system of claim 20 wherein the thirdcurrent source is not more than approximately 10% as large as the secondcurrent source.
 24. The semiconductor laser control system of claim 20wherein the second and third current sources are for capacitivelycoupling to a semiconductor laser.
 25. The semiconductor laser controlsystem of claim 24 further comprising a fourth current sourceproportional to the third current source, the fourth current sourcebeing switchable across the semiconductor laser responsive to the pilottone with a 180 phase shift with respect to the third current source asswitchable across the semiconductor laser responsive to the pilot tone.